A 21-year-old woman with severe physical and mental disability came to an emergency room because of genital bleeding. A spindle-shaped blood clot collected during the medical examination was processed for histology. Microscopic examination revealed a small number of fibrotic chorionic villi surrounded by maternal decidua, blood, and necrosis. Because of the patient's serious psychiatric disability, the medical staff concluded her pregnancy had resulted from sexual abuse; they reported the case to the public prosecutor. The prosecutor ordered DNA analysis on the archival formalin-fixed paraffin-embedded abortion material in order to identify the person who caused the pregnancy. DNA extraction from chorionic villi selectively isolated from the surrounding tissues led to successful STR-typing of fetal cells, which was otherwise prevented by excess maternal DNA. The large number of homozygous genotypes in the fetal profile suggested incestuous paternity. Analysis of reference DNA samples from male relatives excluded the woman's father, paternal grandfather, and maternal grandfather, but paternal alleles of the fetus were constantly present in the genotypes of the woman's brother. 1 table, 1 figure, and 15 references